The present invention relates to an air blowing device for a printing press, which vaporizes dampening water which has spread onto a blanket cylinder through a plate cylinder.
Generally, in an offset printing press, ink that is supplied to the plate surface of a printing plate mounted on a plate cylinder is once transferred to the blanket of a blanket cylinder and then to a printing sheet. At this time, dampening water is supplied in addition to the ink, so the ink attaches only to an image area and not to a non-image area. The dampening water spreads from the plate surface onto the blanket surface and then to the printing sheet sequentially. The printing sheet that has absorbed moisture tends to elongate horizontally and vertically. Particularly, in the case of multi-color printing, each time the printing sheet passes through a printing unit, the printing sheet absorbs moisture and elongates. Hence, a pattern that is printed earlier elongates more to be larger than the original size. Consequently, patterns that should overlap are printed with registration errors to lower the registration accuracy, causing a so-called fan-out phenomenon.
The fan-out phenomenon becomes typical to cause a major printing trouble in a double-sided printing press which prints the obverse and reverse sides of a printing sheet simultaneously, because the moisture to be absorbed doubles. As a countermeasure against the fan-out phenomenon, a method has been proposed which blows air to the outer surface of the cylinder to vaporize the moisture.
Conventionally, as shown in Utility Model Registration No. 2599074, fans which blow air to the outer surface of an ink oscillating roller are arranged on the rear side of a cover that opens/closes the front side of a printing unit. Another arrangement has also been proposed in which air is blown from a plurality of air pipes to the outer surface of a roller which forms an inking device, as shown in Japanese Patent Laid-Open No. 5-169633.
In each of the conventional air blowing devices described above, air is blown in the radial direction of the roller. The air blown to the surface of the roller flows in even to behind the roller to flutter the printing sheet under conveyance or cause a gripping change error of the printing sheet, leading to jamming or a trouble of the printing press. Also, the air that has flown in even to behind the roller to reach the plate cylinder may vaporize the originally necessary moisture on the plate cylinder to degrade the printing quality.
In the latter air blowing device, a plurality of air pipes 43 which blow air toward a blanket cylinder 42 in contact with a plate cylinder 40 and impression cylinder 41, as shown in FIG. 5A, are arranged equidistantly in the axial direction (a direction of arrows A-B) of the blanket cylinder 42, as shown in FIG. 5B. Therefore, portions that are strongly blown with air by the air pipes 43 and portions that are scarcely blown alternate in the axial direction of the blanket cylinder 42. A larger amount of moisture evaporates from the portions that are strongly blown with air. This causes striped density nonuniformities on the printing surface of a printing sheet 44 to degrade the printing quality.